Speech controlled switching arrangement

ABSTRACT

To differentiate speech on an incoming line from control tones, such as engaged tone signals, busy signals, or other non-speech frequencies, so that associated recording apparatus will not be activated by non-speech frequencies but will be activated by speech frequencies, the incoming signal is fed to a discriminator which discriminates on the basis of time dependent frequency variations (i.e. in the intervals between changes in frequency which in the case of speech are different and much shorter than in the case of control tone signals), rather than on the conventional basis of control tones and speech being of different frequencies. Control tones, since they are of substantially constant frequency, even though intermittent and separated by pauses, will not activate an associated switching device to start the operation of the associated recorder. On the other hand, signals of varying frequency or modulated frequency, as in speech, will activate the switching device. This is accomplished in one form of system by passing the incoming signal successively through an amplitude limiter, a discriminator, a separator unit, an amplifier, and a comparator stage, the output of which activates a switching device.

United States Patent [191 Dum1er 1 1 Mar. 25, 11975 [75] Inventor:

[73] Assignee: Compur-Werk Gesellschaft mit beschrankter Haftung & (30.,Munich, Germany 1 Filed: July 30,1973

211 App1.No.:383,650

Related 1.1.5. Application Data {63] Continuation of Ser. No. 206,798,Dec; 10, 1971,

Ernst Dumler, Munich, Germany Primary Examiner-Raymond F. Cardillo, Jr,Attorney, Agent, or Firm-Stonebraker & Shepard [57] ABSTRACT Todifferentiate speech on an incoming line from control tones, such asengaged tonesignals, busy signals, or other non-speech frequencies, sothat associated recording apparatus will not be activated by nonspeechfrequencies but will be activated by speech frequencies, the incomingsignal is fed to a discriminator which discriminates on the basis oftime dependent frequency variations (i.e. in the intervals betweenchanges in frequency which in the case of speech are different and muchshorter than in the case of control tone signals), rather than on theconventional basis of control tones and speech being of differentfrequencies. Control tones, since they are of substantially constantfrequency, even though intermittent and separated by pauses, will notactivate an associated switching device to start the operation of theassociated recorder. On the other hand, signals of varying frequency ormodulated frequency, as in speech, will activate the switching device.This is accomplished in one form of system by passing the incomingsignal successively through an amplitude limiter, a discriminator, aseparator unit, an amplifier, and a comparator stage, the output ofwhich activates a switching device.

10 Claims, 5 Drawing Figures abandoned.

[30] Foreign Application Priority Data Jan. 7, 1971 Germany 2100522 June8, 1971 Germany 2128516 [52] 11.8. CI ..179/6 R, 179/1 VC, 179/100.1 VC[51] Km. C1. H04m 1/64 [581 Field of Search 179/6 R, 100.] VC, 1 VC;340/148 [561 References Cited UNITED STATES PATENTS 2,354,176 7/1944Goldsmith 179/100.1 VC 2,761,897 9/1956 Jones 179/1 VC 3,133,992 5/1964Dickmann 179/6 R 3,433,897 3/1969 Munsen 179/100.1VC 3,671,673 6/1972Uchida I 179/1 VC 3,688,043 8/1972 Konno.... 179/100.1 VC 3,688,1268/1972 Klein 179/1 VC FOREIGN PATENTS OR APPLICATIONS 1,180,789 11/1964Germany 179/1 VC 1 4 m0 5 cm P4 5 5L W J'UU'ULI'U'UL sum 2 5153 O M 8 Qon PATENTEI] MR2 5 I975 Ru 3 an SPEECH CONTROLLED SWITCHING ARRANGEMENTCROSS REFERENCE TO RELATED APPLICATION This application is acontinuation of application Ser. No. 206,798, filed Dec. 10, I971 nowabandoned.

BACKGROUND OF THE INVENTION This invention relates to aspeech-controlled switching arrangement for recording apparatus, and inparticular for telephone answering systems, which discriminates betweenspeech frequencies on the one hand, and predetermined signal frequencies(for example, busy signals, or engaged tones on telephone lines) on theother hand hereinafter sometimes called control tones.

In speech controlled arrangements of this kind, as used for example intelephone answering systems, difficulties may arise because, in additionto the speech frequencies, various signal frequencies may occur in therange of speech frequencies. For this reason, it may happen, in atelephone answering system, that the signal frequencies trigger the sameeffect as the speech frequencies. Thus a signal frequency, for example,an incoming engaged tone may be misinterpreted by an automatic telephoneanswering system as incoming speech, and in some circumstances may useup all or a substantial part of the available storage capacity of therecording device, recording thereon merely the engaged tone or otheruseless information.

From German Offenlegungsschrift No. 1,180,789 (a Germanpatentapplication which is available for public inspection) it is knownto apply the incoming signal in a recording appliance to the input ofamonostable multivibrator which is connected in series with adifferentiator, a rectifier, and a further monostable multivibrator, theoutput signal from the second monostable multivibrator being fed to anintegrating unit after it has been rectified. A voltage proportional tothe frequency of the incoming signal is set up in this integrating unit.An appropriate switch is operated, or not operated, de pending onwhether this voltage lies above or below a predetermined thresholdvalue.

In spite of the considerable outlay required in a speech-controlledarrangement of this character, it is not always possible to achieve asatisfactory solution to the problems which arise. In the first place,this is at tributable to the fact that reaching and surpassing thepredetermined threshold only takes place very gradually as the frequencyincreases, so that the critical switching point is not always absolutelyclear. The main disadvantage of this known arrangement is, however, thefact that it is found impossible to suppress undesirable effects arisingfrom signal frequencies which lie in the medium or upper range of speechfrequency.

In accordance with the present invention, the difficulties and drawbacksof the prior art are overcome by using a discriminator arrangement whichis responsive to time-dependent variations in the frequency of speechsignals, which are present in the case but not for control tones. Theoutput signal from such discriminator is used as the criterion foroperation or nonoperation of a switching device. By discriminating onthe basis of time variations rather than merely on the basis ofdifferent frequencies, the system insures satisfactory and reliablediscrimination between speech and signals, regardless of whether thesignal frequencies lie in the upper, medium, or lower range ofthetransmitted speech frequency band. By the dependent frequency variationsmeant variations in the intervals between the instants oftime when thefrequency ofa signal changes, which variations are not of constantfrequency and thus not independent of time. In speech signals, thefrequency variations are time dependent in that they are of a randomnature and can occur in rapid succession.

Control tones, such for example as an engaged tone, are usually constantfrequencies and do not change in frequency (i.e. do not exhibit the timedependent frequency variations of speech signals). Thus, according tothe present invention, no output voltage, or only an output voltagewhich is very small in comparison to that produced by speechfrequencies, occurs at the output of the discriminator. On the otherhand, speech signals continuously change in frequency and have anadequate time-dependent frequency variation to which, the discriminatoris responsive to provide a large output voltage which results when thespeech signals are present, but not when the control tones occur.

In transmission networks in which very severe fluctuations in level mayoccur between the speech signals on the one hand and the control tones(such for example as engaged tones) on the other hand, it isadvantageous to connect an amplitude limiter before the discriminator inthe comparator connections.

The arrangement or system as a whole may be referred to broadly as acomparator system or comparator arrangement.

In a preferred embodiment of the invention, the arrangement is sodevised that at least two alternately conductive and blocking switchingstages are provided in the comparator system, and they only permitoperation of the switching device when they are simultaneouslyconductive.

In another embodiment of the invention, the arrangement is so devisedthat only impulses at very short intervals resulting from the speechsignals will produce the simultaneously conductive condition of theswitch ing stages for operation of the switching device.

A simple and economical version of the comparator system is obtained, inaccordance with a further feature of the invention, by providing each ofthe switching stages of the comparator system with at least onetransistor, the time constants of these transistors being of differentvalue.

BRIEF DESCRIPTION OF THE DRAWINGS Various embodiments of the inventionare described in more detail below, with reference to the accompanyingdrawings, in which:

FIG. 1 is a block diagram of the complete connections of the apparatusaccording to one embodiment of the invention;

FIG. 2 is a similar block diagram of a second embodiment of theinvention;

FIG. 3 is a wiring diagram of the comparator system; and

FIGS. 4 and 5 are pulse diagrams referring to the comparator system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the firstembodiment of the invention as illustrated schematically in the blockdiagram of FIG. 1, the first element of the monitoring and comparatorsystem is an amplitude limiter 4 which brings the speech signals S andthe control tones B to the same amplitude. At the output of theamplitude limiter 4, the speech signals have the form indicated at SL,while the engaged tones or signals have the form indicated at BL. Itwill be noted that the amplitude limited signals SL are subject totime-dependent variations in that the instants of time when thefrequency thereof changes occur randomly, as shown. In contrast, thelimited engaged tones BL are not subject to time-dependent variations inthat the tones are of a single frequency only.

Connected after the amplitude limiter 4 there is a discriminatorindicated in general at 5, which converts the time of frequency changeof the limited speech signals SL into a signal which varies in amplitudelevel (i.e., has a significant A.C. component) while the limited controltones BL, because of their constant frequency, give no output voltage oronly a very small output voltage which varies in amplitude (i.e. isessentially D.C. without any significant A.C. component). After passingthrough a rectifier stage in the discriminator 5, and a separator unit6, the speech signals take the form indicated at SL', and exhibit alarge amplitude variation with relatively high voltage peaks. Incontrast, the controlled tones, after passing through the separator unit6, are of the form indicated at BL for interrupted engaged tones) or BL"(for continuous engaged tones). Apart from transient phenomena, thelevels of signals BL and BL" are practically constant, or only veryminor extent. In other words the difference between the amplitudes ofthe SL' signals as compared to the BL or BL" signals is significant.

The output of the unit 6 is passed through an amplifier 7 to acomparator stage 8 having a threshold level such that the presence ofspeech signals will be recognized when the threshold level is exceeded,and the presence of control tones, such as include tones and othersignalling, will be indicated below this threshold level. The disturbingeffect of temporary voltage peaks, which may occur for instance at thestart of an engaged tone, may be inhibited by providing a delay circuitwhich will not permit emission of a signal to trigger the recordingmeans when the voltage threshold in the comparator stage 8 is onlyexceeded temporarily.

The threshold level is preferably adjustable. Depending upon whether thethreshold level in the comparator stage 8 is reached (by input ofaspeech signal) or is not reached (by input only of a control tone), therecording means to which the present apparatus is connected will bebrought into operation by a switching device or switching unit 9 whenthere is a speech signal but will be cut out where there is only acontrol tone, and will then be prepared for a fresh incoming call.

Referring now in more detail to the previously mentioned discriminatorunit 5, the output signal from the limiter 4, is applied to adifferentiating circuit consisting of a series capacitor C9 and shuntresistor R10 which differentiates the limited output (SL or BL) into asignal the amplitude of which will fluctuate with time. These amplitudefluctuations (in other words the positive peaks of the differentiatedlimited signals SL and BL) are rectified by the diode D4 and applied toa smoothing capacitor C10. An adjustable resistance or potentiometer P4is connected across the capacitor C10 which provides the output of thediscriminator 5. This output is applied to the coupling capacitor C1] inthe separator unit 6.

The system thus far described in connection with FIG. 1 operates asfollows: If speech signals are fed into the system, the changes infrequency and thus the changes in amplitude of the limited speechsignals SL occur at intervals which vary in duration (see waveform SL ascompared to wave form BL). The thereof voltage produced across thecapacitor C9, and thus across the resistance R10 correspondingly occursat intervals which vary in duration. This voltage is rectified throughthe diode D4, permitting successive charging of the capacitor C10 viathe diode D4 and of capacitor C10 across the potentiometer P4. The levelof the voltage across the capacitor CIO and across the potentiometer P4varies correspondingly (i.e. has a significant A.C. component).Depending upon the regulation of sensitivity by the setting of theadjustable resistance or adjusting potentiometer 14, a correspondinglyvarying proportion (A.C. component) of this voltage at the capacitor C10passes through the coupling capacitor C11 in the separator unit 6, tothe input of the amplifier 7 and'of the threshold level switch 8.

If, in contrast, individual signals of a constant frequency arrive dueto control tones, for example in the form of an engaged tone, thefrequency of the voltage due to the capacitor C9 which appears acrossthe resistor R10 is regularly repetitive so that, after rectificationthrough the diode D4, there is a voltage at capacitor C10 which remainsconstant in amplitude level with relation to time (viz. is essentiallyD.C.). Practically no input voltage will be applied to the thresholdlevel switch 8, in view of the interposed coupling capacitor C11 andbecause there is no change in voltage. In other words the couplingcapacitor C1] functions as such and blocks D.C. levels due to thecontrol tones while passing A.C. levels due to the speech signals.Consequently this switch 8 will not be operated by control tones butwill be operated by speech signals for control tones. Thus there is noinput voltage, or only a voltage which is very slight in comparison withthe threshold at which there is to be a response, and thus there will beno voltage at the switching device 9. The missing signal criterion willthus be met, so that the switching device wil forthwith switch off therecording applicance, because no speech signal is present, but onlycontrol tones.

At times, it may be desired to use a symmetrical push-pulldiscriminator, which may be arranged so that the output voltage afterdemodulation becomes zero for superimposed markings or engaged tones, ifthe frequency thereof is applied to the middle of the discriminatorcharacteristic line as a constant disturbance frequency to besuppressed.

Also, it may be advantageous in many instances to shunt off a secondtransmission line after demodulation and rectification. FIG. 1 showsschematically the use of such a second transmission line, branching offbetween the elements 5 and 6 of the first transmission line. The secondline incorporates an integrating member 10, advantageously in the formof an RC member (resistance capacitance member), along with an amplifier11 and a relay 12. When such a second transmission line is used, it maybe advantageous to connect the output end of this second line to theswitching device 9 which is connected next after the comparator stage 8,and make this switch device operable by signals in the secondtransmission line which are in addition to and independent of thesignals of the first transmission line. This may lead to the facilityfor suppressing disturbances.

Another embodiment of the invention is illustrated schematically in FIG.2. Many parts of this embodiment are similar to those already describedin connection with FIG. I, and require little or no further discussion.

The first element of the system in FIG. 2 is the amplitude limiter 4which brings the speech signals S and the control tones which includeinterrupted engaged tones or other signalling B to the same amplitude.At the output of the amplitude limiter 4, the speech signals have theshape indicated at SL and the engaged tones signals have the shapeillustrated at BL. It will be noted as explained above that the speechsignals SL are subject to frequency variations which charge randomlywith times, that is, are subject to a kind of frequency modulation. Incontrast, the controltones of constant or single frequency and are notsubject to such variations.

The output of the amplitude limiter 4 constitutes the input of thediscriminator 5, which converts the output level which changes in speechsignals SL into an amplitude. Because of the absence of any frequencyvariation i.e., its a significant A.C. component, limited control tonesBL, the discriminator 5 produces an output level which is essentiallyconstant in amplitude (i.e. D.C.) After passing through a rectifierstage in the discriminator 5, and through the separator unit 6, thespeech signals acquire the shape indicated at SL in FIG. 5; that is,they have large amplitude excursions and relatively high voltage peaks.On the other hand, the control tones have the shape indicated at Bl. inFIG. 4, at the output of the separator unit 6. Apart from voltage peaksat the beginning and at the end, the signals BL exhibit no amplitudeexcursion, or only a very slight.

In the discriminator 5, the output signal from the limiter 4 isdifferentiated by the capacitor C9, and resistor R: diode D4 and appliedto a snoothing capacitor C10. These differentiated output signals arerectified by the capacitor C10 is connected across the adjustableresistance or potentiometer P4 which is connected to the couplingcapacitor C11 in the separator unit 6. It will be noted that theseelements operate in the same way as the corresponding elementsin FIG. 1.

The output of the separator unit 6 is fed into the amplifier unit 7, andthe output of the unit 7 is fed into the comparator unit 8, shown ingreater detail in FIG. 3.

This comparator unit or stage 8 includes two switch stages consituted bythe transistors T4 and T5, and corresponding doubling capacitors Cl4/Cl6and C12 which, with the base-emitter resistance ofthe transistor T4,give the different time constants of the two switch stages. The diodesD5 and D6 are included for rectification and voltage duplicationdoubling. Resistances R18, R19 and the base of transistor T4 control thedischarge time of the capacitor C16. The switching device 9 is connectedto the comparator stage 8 at K1 and K2. The switching device operates(i.e., turns on or off) the recording appliance A, for example amagnetic tape recorder. A transistor T3, representing the amplifier unit7, is disposed in advance of the comparator stage 8, as seen in FIG. 3.

The system shown schematically in FIGS. 2 and 3 operates as follows:When speech signals come in the capacitor C9 and resistance R10 providea signal which varies in amplitude and has amplitude excursions whichare time variant in accordance with the frequency changes in the speechsignals. These signals (the limited, differentiated speech signals) thusvary randomly in their occurrence with time. These signals are rectitiedby the diode D4. The capacitor C10 is charged through the diode D4 anddischarged via the potentiometer P4. Depending on the adjustment ofsensitivity by the adjustable or regulatable resistance or potentiometerP4, a correspondingly (A.C.) part of this voltage is applied to thecoupling capacitor C11 which presents an output signal corresponding tothe varying or AC. part of the voltage from the potentiometer P4 (seeFIG. 5) to the input of the amplifier 7, i.e., of the transistor T3.

On the other hand, when the control tones, e.g. due to an engaged tonesignal arrive, a positive impulse is produced at the input of theamplifier-transistor T3 at the beginning of the input of such signalfrequencies, and a negative impulse at the end. These impulses areconditioned by the voltage peaks already mentioned, and have a pulselength of about 20 millliseconds. When, for instance, there is anengaged tone signal of 250 ms. duration and a succeeding pause of about250 ms. duration, the pulse diagram is that illustrated in FIG. 4.Positive impulses are applied at intervals of 500 ms. at the input ofamplifier 7, or its transistor T3, and negative impulses, with a phaseshift of 250 ms., are likewise applied at intervals of 500 ms., thesebeing correspondingly amplified and transmitted to the succeedingcomparator stage 8.

A positive impulse arriving at the comparator stage 8 illustrated inFIG. 3 causes a charging of the capacitor C16, and the transistor T4becomes conductive for a short period of about 50 to milliseconds afterthe threshold level of the transistor T4 is reached. This conductivecondition of the transistor T4 is illustrated in broken lines in thediagram of FIG. 4. In contrast, a succeeding negative impulse causes adischarging of the capacitor C12, and the transistor T5 becomesconductive for a period of 10-20 ms. when the threshold level isreached. The conductive condition of this transistor T5 is illustratedwith dot-dash lines in FIG. 4.

The switch 9 for operating (i.e., for starting) the recording applianceA, which is connected to the termi nals K1 and K2 of the comparatorstage 8, becomes active only when the two transistors stages T4 and T5of the comparator stage 8 are simultaneously conductive. As clearlyshown in FIG. 4, this is not the case when the input is the signalfrequency of an engaged tone signal, because the conductive conditionsof transistors T4 and T5 are produced at intervals one from another, andthe two transistors do not become simultaneously conductive. When speechsignals are received, however, the positive and negative impulses followone another in quick succession timewise, and at intervals of only about10-2O milliseconds. The result of this is that shortly after thecommencement of a speech frequency signal, there is a simultaneousconductive condition of the two transistors T4 and T5 of the comparatorstage 8, as illustrated in FIG. 5, where it is seen that the broken lineimpulse and the dot-dash line impulse overlap each other. As a resultofthe simultaneous conductivity of the transistors T4 and T5, the switch9 responds, and the recording appliance A is turned on to receive thespoken message.

It is to be understood that the invention is not limited to theparticular system of the comparator stage 8 which has been describedabove as an example, but

that various forms of this stage are feasible. For example, the timeconstants of the two switching stages T4 and T5 might even be kept thesame, if the impulses of the two stages are of sufficient duration.

Better results are obtained if the discriminator 5 is a discriminator ofthe type known in the art as a flank discriminator or edge-steepnessdiscriminator. This is a special kind of discriminator (known per se)which measures only the speed of change of an electric pulse, i.e., thesteepness of the edge or flank of the pulse, rather than measuring theheight or absolute value of the pulse.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. Speech-operated switching apparatus for controlling a switch unitadapted to control a recording appliance in a telephone answering andmessage recording system, which apparatus is responsive to input speechsignals and to other input signals of substantially constant frequencyarriving from a telephone line, said apparatus comprising:

a. means responsive to said input signals for converting said inputsignals into signals which change in amplitude,

b. means responsive to the signals from said last named means forproviding output signals which exceed a certain treshold when thefrequency of said input signals changes at a rate corresponding to therate of change of the frequency of said speech signals, and

c. means for operating said switch unit when the interval betweensuccessive ones of said output signals is less than a certain intervalwhich is characteristic of time dependent variations in the frequency ofsaid speech signals.

2. The invention as set forth in claim 1 wherein said means foroperating said switch unit comprises a comparator stage including firstand second transistor stages connected in series, said first and secondstages having R/C input circuits each having a different charging timeconstant, and means for applying said output signals simultaneously tosaid input circuits.

3. The invention as set forth in claim 2 wherein said 'first transistorstage includes a PNP transistor and said being connected to the input ofsaid voltage doubler circuit and to said last-named capacitor.

4. The invention as set forth in claim 1 wherein said converting meanscomprises differentiating means, and wherein said means for providingoutput signals includes:

i. means responsive to the signals from said differentiating means forproviding an output signal level which varies in accordance with thetime variations in the occurrence of said signals from saiddifferentiating means, and

ii. coupling means responsive to said output signal level fortransmitting signals which change in amplitude, said coupling meansproviding said output signals.

5. The invention as set forth in claim 4 wherein said differentiatingmeans is a flank discriminator.

6. The invention as set forth in claim 4 wherein said differentiatingmeans comprises a series capacitor and a shunt resistor across whichsaid input signals are applied; and wherein said output signal levelproviding means comprises a rectifying circuit connected to the junctionof said resistor and capacitor; and wherein said coupling means is acoupling capacitor.

7. The invention as set forth in claim 6 including an amplitude limiterfor converting said input signals into square waves repetitive at thefrequency of said input signals for applying said input signals acrosssaid series capacitor and shunt resistor.

8. The invention as set forth in claim 7 wherein said rectifying-circuitcomprises a rectifier connected to a smoothing capacitor, said couplingcapacitor being connected to said smoothing capacitor.

9. The invention as set forth in claim 8 wherein said means foroperating said switch unit comprises a comparator stage including firstand second transistor stages connected in series, said first and secondstages having R/C input circuits each having a different charging timeconstant, and means for applying said output signals simultaneously tosaid input circuits.

10. The invention as set forth in claim 9, wherein said first transistorstage includes a PNP transistor, the collector emitter paths of saidtransistors being connected in series, means for connecting said seriesconnected paths to said switch unit, a voltage doubler circuit beingconnected to the base of the transistor in one of said first and secondstages, and a capacitor being connected to the base of the transistor inthe other of said transistor stages, said output signal applying meansbeing connected to the input of said voltage doubler circuit and to saidlast-named capacitor.

1. Speech-operated switching apparatus for controlling a switch unitadapted to control a recording appliance in a telephone answering andmessage recording system, which apparatus is responsive to input speechsignals and to other input signals of substantially constant frequencyarriving from a telephone line, said apparatus comprising: a. meansresponsive to said input signals for converting said input signals intosignals which change in amplitude, b. means responsive to the signalsfrom said last named means for providing output signals which exceed acertain treshold when the frequency of said input signals changes at arate corresponding to the rate of change of the frequency of said speechsignals, and c. means for operating said switch unit when the intervalbetween successive ones of said output signals is less than a certaininterval which is characteristic of time dependent variations in thefrequency of said speech signals.
 2. The invention as set forth in claim1 wherein said means for operating said switch unit comprises acomparator stage including first and second transistor stages connectedin series, said first and second stages having R/C input circuits eachhaving a different charging time constant, and means for applying saidoutput signals simultaneously to said input circuits.
 3. The inventionas set forth in claim 2 wherein said first transistor stage includes aPNP transistor and said second transistor stage includes an NPNtransistor, the collector emitter paths of said transistors beingconnected in series, means for connecting said series connected paths tosaid switch unit, a voltage doubler circuit being connected to the baseof the transistor in one of said first and second stages, and acapacitor being connected to the base of the transistor in the other ofsaid transistor stages, said output signal applying means beingconnected to the input of said voltage doubler circuit and to saidlast-named capacitor.
 4. The invention as set forth in claim 1 whereinsaid converting means comprises differentiating means, and wherein saidmeans for providing output signals includes: i. means responsive to thesignals from said differentiating means for providing an output signallevel which varies in accordance with the time variations in theoccurrence of said signals from said differentiating means, and ii.coupling means responsive to said output signal level for transmittingsignals which change in amplitude, said coupling means providing saidoutput signals.
 5. The invention as set forth in claim 4 wherein saiddifferentiating means is a flank discriminator.
 6. The invention as setforth in claim 4 wherein said differentiating means comprises a seriescapacitor and a shunt resistor across which said input signals areapplied; and wherein said output signal level providing means comprisesa rectifying circuit connected to the junction of said resistor andcapacitor; and wherein said coupling means is a coupling capacitor. 7.The invention as set forth in claim 6 including an amplitude limiter forconverting said input signals into square waves repetitive at thefrequency of said input signals for applying said input signals acrosssaid series capacitor and shunt resistor.
 8. The invention as set forthin claim 7 wherein said rectifying-circuit comprises a rectifierconnected to a smoothing capacitor, said coupling capacitor beingconnected to said smoothing capacitor.
 9. The invention as set forth inclaim 8 wherein said means for operating said switch unit comprises acomparator stage including first and second transistor stages connectedin series, said first and second stages having R/C input circuits eachhaving a different charging time constant, and means for applying saidoutput signals simultaneously to said input circuits.
 10. The inventionas set forth in claim 9, wherein said first transistor stage includes aPNP transistor, the collector emitter paths of said transistors beingconnected in series, means for connecting said series connected paths tosaid switch unit, a voltage doubler circuit being connected to the baseof the transistor in one of said first and second stages, and acapacitor being connected to the base of the transistor in the other ofsaid transistor stages, said output signal apPlying means beingconnected to the input of said voltage doubler circuit and to saidlast-named capacitor.